1. Field of the Invention
The field of art to which the claimed invention pertains is solid-bed adsorptive separation. More specifically, the claimed invention relates to a process for the separation of ethylbenzene from a feed mixture comprising ethylbenzene and xylene isomers which process employs a solid adsorbent which selectively removes the xylene isomers from the feed mixture thereby producing a fluid raffinate stream comprising ethylbenzene.
2. Description of the Prior Art
It is well known in the separation art that certain crystalline aluminosilicates can be used to separate hydrocarbon species from mixtures thereof. The separation of normal paraffins from branched chained paraffins for example can be accomplished by using a type A zeolite which has pore openings from 3 to about 5 Angstroms. Such a separation process is disclosed in U.S. Pat. Nos. 2,985,589 and 3,201,491. These adsorbents allow a separation based on the physical size differences in the molecules by allowing the smaller or normal hydrocarbons to be passed into the cavities within the zeolitic adsorbent, while excluding the larger or branched chain molecules.
U.S. Pat. Nos. 3,265,750 and 3,510,423 for example disclose processes in which larger pore diameter zeolites such as the type X or type Y structured zeolites can be used to separate olefinic hydrocarbons.
In addition to separating hydrocarbon types, the type X or type Y zeolites have also been employed in processes to separate individual hydrocarbon isomers. In the process described in U.S. Pat. No. 3,114,782, for example, a particular zeolite is used as an adsorbent to separate alkyl-trisubstituted benzene; and in U.S. Pat. No. 3,668,267 a particular zeolite is used to separate specific alkyl-substituted naphthalenes.
Because of the commercial importance of para-xylene, the more well-known and extensively used hydrocarbon isomer separation processes are those for separating para-xylene. Para-xylene is used in the manufacture of terephthalic acid which in turn is subsequently employed in the manufacture of various synthetic fibers such as Dacron, a trade-marked product of the duPont Company. In processes described in U.S. Pat. Nos. 3,558,732 and 3,686,342 for example adsorbents comprising particular zeolites are used to separate para-xylene from feed mixtures comprising para-xylene and at least one other xylene isomer by selectively adsorbing para-xylene over the other xylene isomers. In such processes the adsorbents used are para-xylene selective; para-xylene is selectively adsorbed and recovered as an extract component while the rest of the xylenes and ethylbenzenes are all relatively unadsorbed with respect to para-xylene and are recovered as raffinate components.
In the process described in our assignee's application Ser. No. 459,251 now U.S. Pat. No. 3,917,734 issued to A. J. deRosset ethylbenzene is recovered in high purity from a feed mixture comprising ethylbenzene and xylene isomers. The process basically comprises contacting the feed mixture with an adsorbent comprising calcium exchanged type X or type Y zeolites, selectively adsorbing the xylene isomers, and thereafter recovering ethylbenzene as a raffinate component. The adsorbent employed is thus all-xylene selective rather than para-xylene selective as are the adsorbents used in the para-xylene separation process. The adsorbed xylenes may then be recovered, in one embodiment, by contacting the adsorbent with a desorbent material, preferably comprising toluene, thereby desorbing the xylenes and then withdrawing the desorbed xylenes from the adsorbent. In another embodiment the adsorption and desorption are done continuously in a simulated moving bed countercurrent flow system the operating principles and sequence of which are described in U.S. Pat. No. 2,985,589. We have discovered that when the feed mixture to this process includes para-xylene and when the preferred toluene desorbent material is employed, the selectivity of that adsorbent is higher for the toluene desorbent material than it is for para-xylene. This results in the inability of that process to obtain high purity product and high yields simultaneously when the ethylbenzene concentration of the feed is about the same as or less than that of para-xylene.
The process of our invention, in one of its embodiments, eliminates that problem. Specifically, we have found that adsorbents comprising strontium exchanged type X or type Y zeolites exhibit selectivity for all the xylene isomers with respect to ethylbenzene and also have the desired higher selectivity for para-xylene than for toluene thereby making separation of ethylbenzene from xylene isomers in both high purity (98% or greater, expressed as a percent of C.sub.8 aromatics present) and high yields (95% or greater) possible for any ethylbenzene concentration in the feed.
Ethylbenzene, used as a raw material in the production of styrene monomer, is commercially produced from the alkylation of benzene with ethylene. The cost of and competing demands for necessary benzene and ethylene feed streams have, however, prompted new efforts to recover ethylbenzene from various C.sub.8 aromatic feed streams which already contain ethylbenzene. Such feed streams for instance include C.sub.8 aromatic extracts produced by a typical solvent extraction process for a pyrolysis gasoline or from a naphtha which has been reformed with a platinum-halogen-containing catalyst. Additionally, C.sub.8 aromatic cuts of hydrogenated pyrolysis naphthas or reformates prepared by fractionation without solvent extraction contain varying amounts of ethylbenzene. The particular utility of the process of our invention therefore is that it offers a method for recovering ethylbenzene from a feed stream which already contains ethylbenzene.
Ethylbenzene can, of course, be separated from the xylene isomers by fractionation but because its boiling point is within about 4.degree. F. of that of para-xylene, the fractionation can be achieved only with the more intricate super-fractionators. Typical ethylbenzene fractionators contain 300 to 400 actual trays and require about a 25-50 to 1 reflux to feed ratio. The process of our invention therefore offers a competitive alternative to the separation of ethylbenzene by super-fractionation.